1. Field of the Invention
The present invention relates to a brake apparatus that performs braking force control of a vehicle.
2. Background Art
Brake apparatuses that perform braking force control having been garnering attention in recent years (see JP Patent Publication (Kokai) No. 9-2222A (1997) (hereunder, referred to as “Patent Document 1”)).
According to Patent Document 1, a braking force is controlled so that a difference between a front wheel velocity (=tire radius×front wheel angular velocity) and a rear wheel velocity (=tire radius×rear wheel angular velocity) matches a target value. In order to stably decelerate a vehicle, it is desirable to make a front/rear braking force ratio (ratio between front wheel braking force and rear wheel braking force) equal to a front/rear load ratio (ratio between front wheel load and rear wheel load), that is, to achieve a so-called ideal braking force distribution (reference literature: “Automotive Technology Handbook (First Volume), Fundamentals and Theory” pp. 141-142 (published by Society of Automotive Engineers of Japan)). According to Patent Document 1, the ideal braking force distribution is achieved by making a target value for a difference between the front wheel velocity and rear wheel velocity 0, and causing the front wheel velocity and the rear wheel velocity to match. This is based on the following principle.
As shown by the following equation, it is known that a braking force F generated by a wheel can be approximated with an equation that is proportional to the product of a wheel load W and a slip ratio S. Here, k is a proportionality constant that is mainly determined according to the type of tire.F=k·W·S  [Equation 1]
The slip ratio S is determined by a wheel travelling velocity V and a wheel velocity (R·ω) (where R is the tire radius, and ω is the wheel angular velocity), and is expressed by the following equation.
                    S        =                              V            -                          R              ·              ω                                V                                    [                  Equation          ⁢                                          ⁢          2                ]            
Since the wheel travelling velocity V is approximately equal at the front wheels and rear wheels, if the wheel velocity (R·ω) becomes equal for the front wheels and rear wheels, then the slip ratio S will also be approximately equal for the front wheels and rear wheels. When the slip ratio S is equal for the front wheels and rear wheels, then based on equation 1, the front/rear braking force ratio is proportional to the front/rear load ratio.